toxi.sim.erosion
Class TalusAngleErosion

For each neighbour it's computed the difference between the processed cell
and the neighbour:

d[i] = h - h[i];

the maximum positive difference is stored in d_max, and the sum of all the
positive differences that are bigger than T (this numer is n), the talus
angle, is stored in d_tot.
Now it's possible to update all the n cells (where d[i] is bigger than T)
using this formula:

h[i] = h[i] + c * (d_max - T) * (d[i] / d_tot);

and the main cell with this other formula:

h = h - (d_max - (n * d_max * T / d_tot));

The Talus angle T is a threshold that determines which slopes are affected by
the erosion, instead the c constant determines how much material is eroded.